Integrated circuits are built in layers, by repeatedly coating silicon wafers with photo-sensitive chemicals, flooding the wafers with a flash of light through a negative image of the circuit pattern, called a mask, and then developing that image and covering the microscopic circuit lines with a thin layer of a metal conductor. Prior to fifteen years ago, when the circuit lines were broader, the image was printed by laying the mask on the wafer. However, as the integrated circuit lines were required to become smaller and smaller, a more sophisticated projection technique was used. The apparatus uses mirrors to focus the image of the mask onto the wafer, and also avoids the need for direct handling and contact with the wafer.
Although projection systems have improved productivity, they still use masks which have the same size as the integrated circuits on the wafer. In order to reduce line widths a stepper alignment system has been developed. This uses a mask which is between five and ten times the size of the chip, which means that the quality of the mask is not as critical as in a projection screen. The mask image is reduced by expensive high resolution lenses and focused on the silicon. However, lenses which are large enough to cover an entire 100 to 125 mm diameter chip do not exist, with the result that the system has to expose the wafer one chip at a time, stepping from chip to chip.
Whilst the stepper system had some advantages in that during the reduction of the mask image, the flaws on the mask were also reduced, with increasing reduction in size of the integrated circuit lines, serious problems arise and specks of dust, minute scratches or fingerprints can result in defective chips.
Thus with serious limitations occuring in the purely optical system, researchers have tried to develop other techniques.
These other techniques in turn have thier own problems. For example, X-ray aligners work like conventional projection gear, but they transmit X-rays, not light through the mask. Since the wavelength of X-rays is considerably smaller than that of light, they can define finer circuit geometries. However, the necessary chemicals needed to record the image are not readily available at a reasonable cost and moreover special masks are required since they have to be capable of stopping X-rays, these special masks being both fragile and expensive.
Another technique which has been tried involves the use of electron-beam exposure systems. However, these types of systems have extremely slow production rates, since they "write" each circuit line separately, as an overlapping series of dots, which is much more laborious than flooding large areas with a flash that instantly inprints a complete pattern. Thus whilst the electron-beam system is capable of producing masks of a high quality, the manufacturing technique required is far too slow to have practical application other than a few specialized fields.
Thus whilst X-ray systems and electron-beam systems do have specific advantages over the optical systems, they do have their limitations. For example, whilst one advantage of the X-ray system is that high resolution can be achieved due to the short wavelength utilized and another is that it is insensitive to soft defects, a new resist system will be required and X-rays are sensitive to proximity effects. Moreover, it will require "stepper" processing with all the problems of throughput and mechanical positioning associated with todays systems. X-ray masks will also require a new mask technology due to the necessity for special mask substrates and gold metallizing.
In recent years, researchers have come back to the possible use of some form of modified holographic system. The problem with the normal optical system is the fundamental physical limit imposed on the system by the wavelength of light which occurs with integrated circuit lines having a width of less then one micron (1.times.10.sup.-6 m). Although it has been proposed that some of the above discussed problems can be overcome by use of a holographic technique, there are certain problems which have to be solved before such a holographic system can become a practical proposition.
One of these problems concerns the use of lasers in the holographic printing technique. Lasers provide coherent light of a given wavelength, whereas ordinary light is "incoherent" being made up usually of a range of wavelengths from the infra-red to the ultra-violet, the light being of radom phase.
It has been observed that when an integrated circuit structure is placed in a laser beam, the resulting image is "speckled", and this problem implies that the circuit images prepared by a holographic technique might have unacceptable flaws.
Another problem concerns the choice of materials for the transparent recording element which would be utilized in any suitable holographic technique. Thus a tried and tested form of holographic medium is in the form of ultra fine grain silver halide. Such a medium would unfortunately suffer from scattering of light from its grainy structure and that phenomenon would almost certainly lead to a loss of resolution of printed images. The most desirable recording medium would be low in scatter and absorption and would almost certainly be grainless. Photo-polymeric media whilst being photographically insensitive would largely avoid the difficulties posed by scatter and absorption. A number of examples of polymeric substances suitable for the process of recording phase holograms have been disclosed by Professor J. J. A. Robillard in British Patent Specification No. 1,471,764.
Given that one can select a suitable medium for recording on the basis of the previous arguments, the fundamental optical limitations of holography can prevent the achievement of the absolute maximum of resolution in the printed image.